In a review of a William Kentridge piece now up at the Met, Holland Cotter says:
"The piece ... has
recently been acquired jointly by the Metropolitan and the San Francisco
Museum of Modern Art. ... [S]urely the nature of the purchase establishes
a salutary practical model, in a time when cash is tight and art prices
sky-high, for institutional resource-sharing in the future."
As I've said before, if it's a salutary thing for Museum A and Museum B to buy work jointly, then it's got to be a salutary thing for Museum A, in a time when cash is tight, to sell a 50% interest in a work (or works) to Museum B. The result is exactly the same. It's a kind of modified Ellis Rule.